Conventionally known is an inverted pendulum vehicle having a saddle unit provided on a base frame including a motion unit for propelling the vehicle on a floor surface and a drive unit for driving the motion unit, and configured to be propelled in a desired direction as commanded by the vehicle operator by leaning toward the desired direction. See JP2011-068165A and JP2013-237325A, for instance.
Such an inverted pendulum vehicle is typically configured to travel faster as the vehicle operator leans more heavily toward the desired direction as long as the leaning angle is within a prescribed range. If the leaning angle is excessive, the drive torque and/or the travel speed that are necessary to control the vehicle in the upright attitude may not be available so that the vehicle operator may have some difficulty in maintaining the upright posture of the vehicle. Therefore, it is necessary to provide a certain mechanism that limits the leaning angle of the vehicle operator within an upright control tolerance range by notifying or warning the vehicle operator. For instance, when the vehicle operator has leaned toward the desired direction by an excessive degree, the vehicle may be tilted in the opposite direction to prevent the leaning angle of the vehicle operator from becoming excessive, or a warning may be issued to the vehicle operator to desist from leaning excessively.
Even when the system warns the vehicle operator not to lean excessively by tilting the vehicle in the opposite direction or issuing a warning, the vehicle may still cause some difficulty for the vehicle operator to maintain the upright posture unless the vehicle operator heeds such a warning and takes an appropriate action. When the vehicle happens to be in a noisy environment, the vehicle operator may fail to notice the warning given as an audible warning.